1. Field of the Invention
The present invention relates to a charged particle beam exposure system that is adapted to continuously scan a number of charged particle beams with a blanking aperture array having a number of rows of open holes.
2. Description of the Related Art
Lately, because of their increasing importance, ICs have been expected to advance in terms of integration density and function as core technology for technological progress of industrial fields including computers, mechanical control, communications, etc. ICs have reached a four-fold level of high integration within the last three or four years. For example, the integration density of the DRAM has increased from 1M through to 4M, 16M, 64M, 256M and 1G.
Said high integration development has depended solely on the advancement of precision fabrication technology, particularly the advancement of optical technology that allows precision operations in 0.5 .mu.m units. However, the precision limit of optical technology is approximately 0.3 .mu.m and it is becoming difficult to maintain a precision of 0.10 .mu.m in windowing for contact holes and alignment with a pattern of a lower layer. Since a pellicle cannot be applied to a mask in the case of X-ray exposure and a defect-free guarantee is difficult, X-ray exposure cannot be used in the 35 fabrication of LSIs that are required to provide high reliability.
Though, in charged particle beam exposure, precision operations of 0.10 .mu.m or under can be achieved with alignment accuracy of 0.05 .mu.m. It has been considered that charged particle beam exposure could not be used for volume production of LSIs because of low throughput. In this case, a one-line configured pattern exposure system has been assumed for precision pattern exposure with at most only one Gaussian beam or variable square beam. The above assumption has been determined in view of the productivity of the existing commercial systems, and not based on the results of checking physical and technological bottlenecks and clarification of causes as to why the said throughput could not be increased or on any consideration as to how the throughput could be increased.
Lately, however, the inventions of a block exposure system and a blanking aperture array system by the inventor of the present invention and others have made it possible to expect a throughput of approximately 1 cm.sup.2 /sec. This exposure system is so advantageous that other lithographic means cannot compare in terms of precision, alignment accuracy, quick turnaround, reliability and advancement of related software. Thus, the charged particle beam exposure system that enables the manufacturing of nano-lithographic LSIs, such as 1 to 4 GBIT memories, is considered to be a prospective lithography system.
The most popular type of charged particle beam exposure system is the point beam raster scan exposure system. However, in this system, only one beam is used and the throughput is extremely low and therefore the exposure of wafers is impossible at a volume production level.
On the contrary, in the case of the variable square beam system, a limited size of square can be formed by one shot and therefore a satisfactory throughput can be obtained with a rough pattern, the smallest pattern of which is approximately 2 .mu.m. Accordingly, this system can be used for direct exposure production of small lots of products. However, LSIs of high pattern density cannot overcome the restriction of a one-writing pattern and cannot avoid an extremely low throughput.
As a supplementary system for the above, a system for transferring a repetitive memory pattern as a contraction image of charged particle beams passing through a silicon stencil mask is referred to as the block exposure system. This block exposure system allows volume production of memory chips of 256M and 1G by repeatedly irradiating a pattern, which is used repeatedly for the memory, at a high repetition rate.
However, the block exposure system has the vital defect that the throughput for a random pattern is extremely low, as in case of the variable square beam. Most gate arrays and micro computers are often based on random patterns.
As a method for high speed exposure of such random patterns, a method has been proposed using line beams with the blanking aperture array. FIG. 14 shows an example of a configuration of the prior art blanking aperture array, and this blanking aperture array 6 is provided with a row of openings 62 that are a plural number of apertures arranged in a line. An ON or OFF signal from the beam ON/OFF signal generator 33 is applied independently into the electrodes 61, disposed on the inside surface of the corresponding openings 62, through respective circuit lines l.sub.1, l.sub.2, . . . l.sub.n and a constant voltage (for example, a ground potential) is applied to the electrodes 63 provided at the other (i.e., opposite) inside surface of the corresponding openings 62 through a common line (not shown).
Thus, a plurality of beams passing through the openings 62 are arranged in a line and these blanking apertures 62 are ON/OFF controlled by an appropriate control means.
In other words, the system is adapted so that the charged particle beams that have passed through the blanking apertures 62, into which the ON signal is applied, reach the specified point on the surface of object to be exposed and the specified point is exposed and the charged particle beams that have passed through the blanking apertures 62 into which the OFF signal is applied and are interrupted by an appropriate shield plate and thereby prevented from reaching the surface of object to be exposed. Then a specified pattern were exposed as if it is applied by a brush, by scanning the surface with a plurality of charged particle beams obtained by the above described ON/OFF control.
For volume production by direct exposure of wafers, it is necessary to expose a 1 cm.sup.2 segment of wafer within one second or, at longest, two seconds. If the throughput is determined as described above, the second problem is the sensitivity of resists. Electrons are particles and the number of electrons that are incident into a unit area within a unit time varies in terms of the Poisson's distribution and therefore the resolution is basically in inverse proportion to the square root of the sensitivity. Accordingly, if the smallest pattern is 0.2 .mu.m, a resist with the sensitivity of 5-10 .mu.C/cm.sup.2 is generally required. A high sensitivity resist has only a low resolution and the system for which implementation of high sensitivity is excessively expected cannot be used in actual volume production of LSIs.
Assuming a throughput of 1 cm.sup.2 /sec on a target value for the resist sensitivity of 5-10 .mu.C/cm.sup.2, the required overall current is 5-10 .mu.A. Another beam size is assumed as 0.05 .mu.m.quadrature. on the surface of a specimen. The limit of the thermal electron gun of LaB.sub.6 is a current density of 250 A/cm.sup.2 to be obtained using a lens of an extremely small spherical aberration and chromatic aberration factor. Accordingly, the current density of 0.05 .mu.m.quadrature. is 250 A/cm.sup.2 and therefore the current value of one beam is 250.times.(0.05.times.10.sup.-4).sup.2 =6.25 nA. The overall current value of 1600 beams is 10 .mu.A.
The problems in the case of exposure of the specified pattern with the above described line beams (for example, 1600 line beams) using the blanking aperture array as shown in FIG. 14 are as follows:
(1) A coulombic interaction has been considered a physical bottleneck for the charged particle beams. This coulombic interaction is a phenomenon such that the beams become dim because of interactive repulsion of electrons. The main cause of this phenomenon is that the focal distance is extended because of interactive repulsion of charged particle beams in proportion to the current value of overall electron flow in the lens near the surface of a specimen (in other words, in proportion to the number of charged particle beams in the ON state) and the focal point deviates downwardly from the specimen surface (that is, the surface of wafer 19) as shown in FIG. 7 (B).
In FIG. 7 (B), numeral 8 denotes the charged particle beam in the ON state and numerals 12 and 17 denote the electron lens for focusing as in the apparatus shown in FIG. 12. The focal deviation due to such coulombic interaction scarcely takes place when the number of charged particle beams 8 in the ON state is small, as shown in FIG. 7 (A), and instead occurs when the number of charged particle beams 8 in the ON state is large (the maximum number of beams reaches, for example, 1600 as described above), as shown in FIG. 7 (B).
The focal deviation because of said coulombic interaction can be corrected by providing a small refocus coil (for example, approximately 4 mm in diameter) at a position near the peak of the magnetic field in the final stage lens or the preceding lens (that is, a position where the magnetic field is most intensified) and supplying a refocus current in proportion to the current value of all charged particle beams (that is, the number of charged particle beams that remain in the ON state at that time) to the refocus coil at the rate of, for example, 50 nsec (50+1 sec). In this case, the rate of approximately 50 nsec is the limit of the response speed of the amplifier (analog current-driven type amplifier) for supplying the specified refocus current to the refocus coil in accordance with the number of charged particle beams that remain in the ON state at that time.
If the size of one beam is set to 0.05 .mu.m.quadrature. and the scanning speed is set to 100 .mu.m/5 .mu.sec (that is, 0.05 .mu.m/2.5 nsec) in the stage moving direction by using the line beams through the blanking aperture array as described above, the beam dwell time per 0.05 .mu.m (one shot of irradiation beam) is 2.5 nsec. Since such line beams are scanned at a high speed, it is necessary to vary the refocus current (for example, approximately 1A, maximum) in steps (for example, 0A to 1A) within a time far shorter than 2.5 nsec, which is the beam dwell time at the above-described point at the boundary between the exposure of a completely written-out region and the partially exposed region. However, it is impossible to obtain said response speed even with the advancement of analog current-driven type amplifiers, which have been developed recently. Accordingly, as shown in, for example, FIG. 15, for a pattern having a small projection (in other words, when the pattern B is exposed, only two beams of the line beams become ON as shown with black dots 10B and 11B in the pattern B) adjacent to a large written-out pattern A (in other words, when the pattern A is exposed, all beams arrayed in a line become ON as shown with black dots 1B, 2B . . . nB in the pattern A) and for the scanning direction of the line beams shown by the arrow in FIG. 15, it is difficult to supply a correct refocus signal as described above (for supplying a correct refocus signal, the current to the refocus coil should be made to rise from approximately 0A to, for example, 1A within a far shorter time than, for example, 2.5 nsec but it is impossible as described above). If the number of line beams is increased in the charged particle beam exposure by one line beam as shown in FIG. 14, the focal deviation due to the above described coulombic interaction cannot be corrected and therefore the precision pattern writing cannot be carried out for volume production.
(2) Since the beam dwell time of 20 nsec is required for the resist sensitivity of 5 .mu.C/cm.sup.2, a current density of 250 A/cm.sup.2 is insufficient for the beam dwell time of 2.5 nsec described above. A current density of 2000 A/cm.sup.2 is required to obtain the dwell time of 2.5 nsec and therefore general thermal electron guns and electron lenses cannot attain this level of current density.
(3) An intermediate color cannot be represented with the conventional line beams and a pattern of dimensions as large as an integral multiple of the dimensions of a single beam cannot be formed. Though such a pattern can be formed by plural times of movement of the stage or beam scanning, the throughput is extremely low and volume production cannot be carried out.
(4) The amount of exposure with the conventional line beams cannot be reduced by a proximity effect correction.
(5) If the conventional line beams are arranged to reduce the effect of the electric field of the openings adjacent to the blanking aperture array, the irradiation points become vacant and therefore the scanning of a plurality of line beams that are slightly deviated is repeated and the throughput will be reduced. In other words, the problems associated with a blanking aperture array that comprises the above described plurality of blanking apertures 62 arranged in a row can be summarized as described below.
The exposure efficiency with the charged particle beams can be improved by using the blanking aperture array, whereas a resist that provides a higher resolution should be used in accordance with the degree of precision of a pattern to be exposed and therefore the exposure time becomes longer and the throughput is reduced.
In the blanking aperture system, there is a problem in that the lens system is affected by the total amount of charged particle beams that pass through the above described blanking apertures. Therefore the focal point deviates and the pattern is dimmed. When exposing, for example, a pattern as shown in FIG. 15, there is a problem in that the total amount of charged particle beams passing through the blanking aperture array substantially varies in the exposure of the B row pattern after exposure of the A row pattern. In addition the focal distance of the charged particle beam exposure system deviates significantly. Therefore, a considerably large current is required and it takes a certain period of time for correction.
For this reason, it is necessary to provide a special control adjustment circuit.
In addition, in the blanking aperture system, the exposure should be carried out through the blanking apertures that are arranged as close to each other as possible to prevent such defects as deformation or discontinuity of the pattern or the like. However, the charged particle beams have a fixed intensity distribution as described above. Therefore there is the so-called proximity effect that adjacent charged particle beams interfere with each other and the pattern is exposed beyond the predetermined amount of irradiation. For example, it is necessary to adjust the amount of irradiation of the charged particle beams at the center and both ends of the blanking aperture array. However, such adjustment has been difficult in the conventional blanking aperture array.
In some cases, it is necessary to vary the amount of irradiation on part or all of the pattern, depending on the shape of the pattern or the relative position of the pattern to other patterns, but such variation cannot be implemented by the conventional blanking aperture array.
Some examples of two-dimensional arrangement of the groups of blanking apertures in the above described blanking aperture array unit have been known to solve the above problems. In the blanking aperture array unit provided with the blanking apertures that are two-dimensionally arranged as described above, all charged particle beams forming the specified pattern passing through the blanking aperture array unit are simultaneously irradiated onto the specified position of an object to be exposed to carry out an exposure process and subsequently all charged particle beams formed in the same or different pattern are simultaneously irradiated onto an adjacent object to be exposed to carry out the exposure processing. If the total amount of charged particle beams passing through the blanking apertures of the above described blanking aperture array unit largely differs with each pattern, a refocusing problem as described above occurs, the proximity effect problem is not resolved and there is no possibility of varying the amount of irradiation onto part of the specified pattern.
Further, a high speed pattern forming method is disclosed by U.S. Pat. No. 4,153,843 in which a blanking aperture array provided with a plurality of blanking apertures arranged in two dimensional form is used. Thus, a plurality of charged particle electron beams are radiated on a surface of a sample to be exposed to the beam to form a predetermined pattern thereon, and a total amount of radiation value is received at a certain addressed position of the sample to which a respective charged particle electron beam is to be exposed.
In the method disclosed in the U.S. Pat. No. 4,153,843, as shown in FIG. 20, each one of the charged particle electron beams formed by each one of the apertures A, D, and G forming a first aperture line arranged parallel to a scanning direction Y of the charged particle electron beams radiates each one of addressed positions of the sample forming one line 1 arranged parallel to the scanning direction Y of the charged particle electron beams. Each one of the charged particle electron beams formed by each one of the apertures B, E, and H forming a second aperture line arranged parallel to a scanning direction Y thereof radiates each one of addressed positions of the sample forming another line 6 arranged parallel to the scanning direction Y thereof. Each one of the charged particle electron beams formed by each one of the apertures C, F, and I forming a third aperture line arranged parallel to a scanning direction Y thereof radiates each one of addressed positions of the sample forming another line 11 arranged parallel to the scanning direction Y.
The charged particle electron beams formed by apertures forming respective lines radiate addressed positions formed on any one of the lines on the sample, and each line is separated by a predetermined space from each other.
Accordingly, the spaced addressed positions are successively radiated by each one of the charged particle electron beams formed by each one of the aperture lines formed in the aperture array and thereby all addressed positions are ultimately fully exposed; this method is generally called an interleave method.
Thus, the charged particle electron beams formed by the aperture A exposes sequentially, continuously arranged addressed positions to form in turn a line on the samples, (1, a), (1, b), (1, c) . . . (1, 1), (2, 1), (2, k), . . . (2, a), (3, a), (3, b), . . . (3, 1).
On the other hand, the charged particle electron beams formed by the aperture B exposes sequentially, continuously arranged addressed positions to form in turn a line on the samples, (6, a), (6, b), (6, c) . . . (6, 1), (7, 1), (7, k), . . . (7, a), (8, a), (8, b), . . . (8, 1).
Further, the charged particle electron beams formed by the aperture C exposes sequentially, continuously arranged addressed positions to form in turn a line on the samples (11, a), (11, b), (11, c) . . . (11, 1), (12, 1), (12, k), . . . (12, a), (13, a), (13, b), . . . (13, 1).
The characteristic feature of the conventional technology is such that a plurality of addressed positions formed on the sample, each of them being separated a predetermined distance from each other, are simultaneously and respectively exposed to different charged particle electron beams in a predetermined exposed region.
In the conventional technology, the charged particle electron beams formed by the apertures D and G in the first aperture line formed by the apertures A, D, and G expose the same addressed position to which the charged particle electron beam formed by the aperture A exposes successively with a predetermined time difference. Therefore, in the embodiment as shown in FIG. 20, one of the addressed positions in the sample to be exposed will be exposed three times by the charged particle electron beam and therefore, a dose value of the same addressed position caused by the exposure of the beams can be optionally changed by changing the exposure number within 3 times. The conventional technology, however, discloses that each of the charged particle electron beams exposes each one of the addressed positions respectively and thus control of the beam is difficult.
Therefore, a problem arises in that when the beam exposes all of a predetermined region to be exposed, a disconnected portion among the pattern to be formed will appear because of a displacement of the beam from an addressed position to be correctly exposed. The disconnected portion should have been connected.
Accordingly, in the conventional technology, since the charged particle electron beams expose addressed positions separated from each other, an aberration and a reduction ratio of each one of the charged particle electron beams changes because of the difference in the distance from the optical axis. Thus, another technology is required to adjust the displacement. Further in the conventional technology, since the controlling operation for controlling the deflection of the beam to realize the interleave operation becomes difficult, the beam controlling circuit becomes complicated.
Moreover, in the conventional technology, a predetermined time should be required from the time when the beams have exposed one addressed position to the time when the same beams expose another adjacent addressed position.
Generally, a light sensitive layer formed at a predetermined addressed position is affected by another beam that exposes another addressed position next to the predetermined addressed position. Thus in the conventional technology, since the exposure time is different, the exposed condition of the sensitive layer of one addressed position will change as time elapses.
Accordingly, it becomes difficult to maintain the light sensitive condition of the predetermined addressed position at the predetermined level, and a uniform exposure operation cannot be expected.